1. Field of the Invention
The present invention relates to a photoelectric converter and more particularly, to a photoelectric converter used as an input unit of a facsimile system, an image reader, a digital copying machine, an electronic blackboard or the like.
2. Related Background Art
In recent years, an elongated line sensor including an optical system having a 1:1 magnification has been developed as a photoelectric converter for a compact, high-performance system such as a facsimile system and an image reader. However, various problems are presented in the development of a compact, high-performance system as follows.
A typical photoelectric converter is exemplified by an image sensor represented by a contact type line sensor used in a facsimile system, an image scanner or the like. The contact type line sensor includes a reading unit having the same size as that of a width of an original. A conventional image sensor of this type is manufactured or formed using a photoconductive material such as hydrogenated amorphous silicon (to be referred to as a-Si:H hereinafter), CdS, and CdSe on an elongated substrate consisting of glass or the like according to a thin-film deposition technique such as deposition.
The assignee of the present applicant proposed a photoelectric converter in Japanese Patent Application No. 61-144990. In this photoelectric converter, thin-film transistors (to be referred to as TFTs hereinafter) of a-Si:H or poly-Si, TFT sensor elements, storage capacitors, and the like are formed on a single substrate.
FIG. 1 shows an equivalent circuit diagram of a matrix driven image sensor.
The image sensor includes TFT sensors S.sub.1 to S.sub.n.multidot.m storage capacitors C.sub.s1 to C.sub.sn.multidot.m for storing photocurrents from the sensors S.sub.1 to S.sub.n.multidot.m switches (TFT) U.sub.1 to U.sub.n.multidot.m for resetting the charges of the storage capacitors C.sub.s1 to S.sub.sn.multidot.m and switches (TFT) T.sub.1 to T.sub.n.multidot.m for reading out the charges from the storage capacitors C.sub.s1 to C.sub.sn.multidot.m and transferring them to readout capacitors C.sub.L1 to C.sub.Ln. The reset and transfer TFTs are blocked in an n.times.m matrix. The image sensor also includes a shift register/driver 11 for sequentially selecting and driving gate lines (1 to m+1) and a switch array 12 for reading voltages of signal lines (1 to n) and serially converting and outputting the read voltages.
FIG. 2 shows a one-bit wiring pattern of the image sensor of the equivalent circuit diagram shown in FIG. 1. The one-bit wiring pattern includes a signal line matrix unit 13, a sensor 14, a contact hole 15, a storage capacitor 16, a transfer TFT 17, a reset TFT 18, a gate wiring unit 19, and an illumination window 20. In this arrangement, light passing through window 20 from below is reflected by an original and received by the sensor, thus constituting a "lensless" type arrangement.
FIG. 3 is a sectional view of the pattern of FIG. 2 along the line X-X' of FIG. 2, FIG. 4 is a sectional view thereof along the line Y-Y' of FIG. 2. The pattern in FIG. 3 comprises a glass substrate 1, a lower electrode 21, an insulating layer 3, an a-Si:H layer 4, an n.sup.+ -type doped layer 5, and an upper electrode layer 22. Similarly, the pattern in FIG. 4 comprises a glass substrate 1, lower electrodes 21, an insulating layer 3, an a-Si:H layer 4, an n.sup.+ -type doped layer 5, and upper electrode layers 22 and 23.
In the conventional patterns, since the TFT sensor is used, photosensitivity is as large as 10 to 100 times that of a photodiode type sensor. In addition, the above pattern has advantages in a high light transmission rate and high illumination on the sensor surface.
However, since a sensor photocurrent is large, a large storage capacitance corresponding to the sensor photocurrent is required. Although the large storage capacitance is advantageous from the viewpoint of characteristics such as an S/N ratio and the dynamic range, the large storage capacitance is not necessarily advantageous from the viewpoint of manufacturing cost. More specifically, when the storage capacitance is increased, the substrate size is increased by an area corresponding to an increased capacitance component. Therefore, the number of sensors per batch is undesirably decreased.
In the elongated line sensor including the optical system having a 1:1 magnification, signal processing ICs (Integrated Circuits) constituted by switching elements or the like are connected to an array of photoelectric conversion elements. However, the number of photoelectric conversion elements is 1728 in an A4 size when the photoelectric converter complies with the facsimile G3 standard. A large number of signal processing ICs are thus required. For this reason, the number of mounting steps is increased, the manufacturing cost is increased, and sensor reliability is degraded. Matrix wiring is employed to reduce the number of signal processing ICs and the number of mounting steps.
FIG. 5 is a block diagram of a photoelectric converter employing matrix wiring. Referring to FIG. 5, the photoelectric converter includes a photoelectric conversion unit 201, a scanning unit 201, a signal processing unit 203, and a matrix wiring unit 204. FIG. 6 is a plan view of the conventional matrix wiring unit, and FIGS. 7A and 7B are sectional views of the matrix wiring unit of FIG. 6 along the lines A-A' and B-B', respectively.
Referring to FIGS. 7A and 7B, the matrix wiring unit includes a substrate 301, separate electrodes 302 to 305, an insulating layer 306, common lines 307 to 309, and a through hole 310 for bringing the separate electrode into ohmic contact with the corresponding common line.
In the photoelectric converter employing matrix wiring, the number of signal processing circuits in the signal processing unit 203 is equal to that of output lines of the matrix. Therefore, the signal processing unit can be advantageously made compact and the photoelectric converter becomes inexpensive.
In the photoelectric converter using a thin-film semiconductor, sensor elements and TFTs constituting a transfer circuit are simultaneously formed on a single substrate, thus providing a compact, low-cost photoelectric converter. In addition, still another conventional photoelectric converter is also proposed wherein photoelectric conversion elements detect, through a transparent spacer of glass or the like, light reflected by an original. A 1:1 magnification fiber lens array is not used in this arrangement.
FIG. 8 shows a illustrative sectional view of such a photoelectric converter. At a mount base 175, a light incident window 178 is provided. A light from a light source 174 shines through the light incident window 178. On the mount base 175, a photoelectric converter 173 having photoelectric conversion elements (not shown) is disposed adjacent a thin transparent spacer 177 which is adjacent to the original. IC176 is provided for processing a light signal from the photoelectric conversion device 173. The light passing through the incident light window illuminates, via the photoelectric conversion device 173, the original 178 fed by a paper feed roller 171. A light reflected from the original is read by the photoelectric conversion elements on the device 173.
The photoelectric converter using the conventional matrix wiring has the following problems.
Since a very low output of each photoelectric conversion element is read out the through matrix wiring, crosstalk occurs between output signals unless a stray capacitance at an insulated (overlapping, non-contact) intersections between the separate output electrode of each photoelectric conversion element and the corresponding common line of the matrix is sufficiently decreased (see the non-contact wiring intersections in gate wiring unit 19 in FIG. 2). This condition imposes strict limitations to the selection of an insulating interlayer material and dimensional design of the matrix.
Each matrix common line extends in the longitudinal direction of the photoelectric converter and undesirably has a length of, e.g., 210 mm in a line sensor having a length corresponding to an A4 width. For this reason, when a capacitance between the common lines is not sufficiently decreased, crosstalk occurs between the output signals. This disadvantage results in a large matrix unit.
The pitch of the separate output electrodes of the photoelectric conversion elements is as small as, e.g., 125 .mu.m in a photoelectric converter having a resolution of 8 lines/mm. For this reason, crosstalk occurs between the output signals unless the capacitance between the separate electrodes is sufficiently decreased.